Provide an API to dart:developer to control the web server hosting the Service Protocol

- [x] Add `ServiceProtocolnfo` class to dart:developer.
- [x] Add `Service` class to dart:developer.
- [x] Add `Service.getInfo` static method to dart:developer.
- [x] Add `Service.controlWebServer` static method to dart:developer.

API:

```dart

/// Information about the service protocol.
class ServiceProtocolInfo {
  /// The major version of the protocol.
  final int majorVersion;
  /// The minor version of the protocol.
  final int minorVersion;
  /// The Uri to access the service. If the web server is not running, this
  /// will be null.
  final Uri serverUri;
}

/// Access information about the service protocol and control the web server.
class Service {
  /// Get information about the service protocol.
  static Future<ServiceProtocolInfo> getInfo();

  /// Control the web server that the service protocol is accessed through.
  static Future<ServiceProtocolInfo> controlWebServer({bool enable: false});
}
```

... and add a randomly generated authentication token path prefix that must be passed in to access the service protocol.

Old base url:

Observatory listening on http://127.0.0.1:54804/

New base url:

Observatory listening on http://127.0.0.1:54804/<token>/

For example:

Observatory listening on http://127.0.0.1:54804/PTwjm8Ii8qg=/

Many tools will need to be updated.

Fixes #23320

BUG=
R=asiva@google.com, rmacnak@google.com

Review URL: https://codereview.chromium.org/2438613002 .